a. Field of the Invention
The invention relates to an interferometric system for measuring the point-for-point distance deviation of an unknown surface from a known reference surface. The invention features a surface or a wavefront measuring interferometer system with a large measurement dynamic range. In particular, it deals with the method and apparatus for measuring optical path difference of a wavefront reflected from an unknown surface figure from that of a wavefront reflected from a known reference surface.
b. Prior Art
The use of interferometric systems for surface and wavefront measurement is well known in optics. In most of the automated interferometric surface measurement systems, the geometry and the shape of the fringes produced are analyzed to obtain the required measurement data. Such measurement systems are particularly useful for measuring and testing aspheric lens surfaces. In recent years there has been considerable interest in making use of complex and higher order aspheric surfaces in optical system design.
Aspheric lenses are well known in optics. For many years, simple aspherics, such as parabolic surfaces, have been used in telescopes. More recently, computer controlled lens grinding machines have been able to produce complicated aspheric lens surfaces. Some of these complicated aspheric surfaces are for spatial compression of multi-element convex-concave lens system. For example, in microscopes and cameras it has been the practice in the prior art to achieve desired corrections and magnifications by means of groups of lenses having specified concave or convex lens curvatures, refractive powers and spacings. Now, however, it is possible to produce a single aspheric lens which will replace a group of lenses.
When testing such surfaces using conventional interferometers, the density of the fringe pattern becomes too high and their shapes too complex to handle. A lack of good and simple means of testing such complex surfaces has been somewhat responsible for the delay in their widespread use. Even though the desired shape of a lens may be specified, it is difficult to determine when a precision surface has been attained. A precision surface is one having a point-for-point accuracy of the order of a fraction of a wavelength. The object of the present invention is to devise a method and apparatus for the testing of complex and higher order aspherical optical surfaces which are impossible to test based on existing testing methods.
In U.S. Pat. No. 4,022,532, issued May 10, 1977, to Montagnino teaches use of a dual beam laser interferometer for comparing phases of multiple reflective spots on a test object. A reference beam, whose path length is modulated, is combined with the light reflected from the spots. Separate detector elements measure the interference pattern from at least two spots simultaneously. A shift in position of one spot relative to a reference spot is determined by measuring the phase shift between spots. Using this approach, the surface configuration of an optical surface may be monitored.
In U.S. Pat. No. 3,694,088, issued Sept. 26, 1972 Gallagher et al. teach use of a dual beam laser interferometer for the study of intensity changes in a fringe pattern by means of a TV camera. The pattern intensity is changed twice by rotation of a quarter-wave plate, producing two known phase shifts. By storing pattern intensity values before and after the rotations, the intensity values can be correlated with the phase shift to solve simultaneous equations which yield phase and amplitude plots for the wavefront from the object under study.
In my prior patent application Ser. No. 912,212, now U.S. Pat. No. 4,225,240, an interferometric method is disclosed for measuring the optical path difference between a test surface and a reference surface. The method consists of varying the interferometric optical path length difference between a reference and a test surface in three steps at one-quarter wavelength intervals. Next, the intensity of the interferogram radiation is sensed at least at one position of the interferogram for each of the steps. The intensity sensed at each position and at each step is stored. For each of the positions the intensity of the first and third steps is added to produce a d.c. spatial frequency amplitude, and the intensity of the second step is subtracted from the d.c. amplitude to produce the sinusoidal spatial frequency amplitude. The sinusoidal and cosinusoidal amplitudes are combined to produce a trigonometric function of the phase angle of the radiation reflected from each position of the reference and test surfaces. This function is representative of the optical path length difference at each position. A multi-aperture CCD detector is used to detect intensity changes of the fringes. An advantage of my prior invention is that the sign of optical path differences may be determined, depending on whether the d.c. amplitude is larger or smaller than twice the intensity at the second step.
In the book "Optical Shop Testing" by Malacara (Wiley, publisher), p. 17 a procedure is described for determining the deviation of an aspheric surface from a spherical surface or an irregular surface from a reference flat surface. One surface is placed atop the other so that an optical path difference between the two will produce fringes when illuminated by a monochromatic source.
In an article entitled "Quasi-Real-Time High Precision Interferometric Measurements of Deforming Surfaces" in SPIE, Vol. 153 (1978) p. 126, Massie describes a system wherein two beams with orthogonal polarizations are shifted in frequency by different amounts using acousto-optic devices. The reference surface receives one polarization and frequency and the test surface the other. With appropriate optics the phase of one beam is compared to the other so that optical path differences can be mapped.
In an article entitled "Automatic Data Reduction of Both Simple and Complex Interference Patterns" in SPIE, Vol. 171 (1979) W. Augustyn discloses a computer fringe pattern analysis method whereby points on a reference interferogram representing zero path difference are placed in memory. Next, a test interferogram is generated and the stored points are subtracted from the actual. The difference between the two patterns is a new interferogram for user study.
White light interferometry has also been used for monitoring surfaces and surface profiles, but its application has been limited to interferometric objective lenses. The use of white light enables one to identify a zero-order fringe as the white light fringe and hence permits quantitative, but manual reduction of interferograms. This is extremely important when surface discontinuities are involved. Several microscope objectives that are capable of producing white light fringes on micro specimens are commercially available and they are typically used for measuring the film thicknesses and monitoring surfaces with discontinuities several wavelengths deep. Unlike other interferogram analyzers cited earlier no attempt has been made to automate the detection and interpretation of white light fringe patterns.
An object of the invention is to provide a simple and direct method for precision characterization of unknown surfaces which does not require visual fringe interpretation and which is suited to the measurement of discontinuous and steeply contoured aspheric surfaces.